Patch notes are available on the Nintendo Switch. Most notably, the online matchmaking will now give preferred rules a higher priority than before. The downside to this is that there might be longer waiting times between matches.
It also specifically says that there still isn't any guarantee that you'll find a match that has your exact preferred rules. Given, this might only apply to very wonky rules that nobody else is searching for.
Interestingly enough, balance adjustments have already been made. The list of adjusted fighters are Donkey Kong, Link, Kirby, Luigi, Ice Climbers, Young Link, Olimar, Toon Link, Villager, Greninja, PAC-MAN, Duck Hunt, and Isabelle.
Unfortunately, no exact details were provided about how these characters were changed. It's unknown if they received buffs or nerfs.
The difficulty of unlocking challengers has also been adjusted. While it doesn't specifically say how, it's likely that the AI has been toned down in order to make the unlocking process a little easier.